In many industries, particularly in the aircraft and transportation industries, there is considerable impetus for the reduction of weight of vehicle components. In many cases, for example, these reductions in weight are necessary to achieve designated fuel economy standards which are becoming ever more stringent. Thus, it has become common within the automotive sector as well as in other transportation industries, to consider alternative designs of many vehicle components, even when the alternative designs incur a cost penalty, if the resulting parts can achieve significant weight savings.
There are many parts for which weight savings are desired. For example, in the automotive industry load floors and seatbacks are but two of such items. Load floors are essentially planar structures of fairly large areal dimensions which are placed over cargo holds, spare tire recesses, and the like. Since these floors must not overly flex upon the addition of a cargo load to the vehicle, or by the presence of vehicle occupants over this area, these floors must have appreciable stiffness. However, current floors, in order to achieve this required stiffness, are made of relatively thick section, dense materials which do not lend themselves to weight savings. Likewise, in the case of seatbacks, the relatively large buckets that surround many seats would desirably be produced in lighter weight versions without losing their structural capabilities. In the non-automotive industries, articles such as molded seats, garage doors, and the like are also amenable to use of lightweight, yet strong and highly stiff materials.
In the past, when high stiffness, high modulus materials have been utilized, they have often been prepared from substrates such as aluminum or thermoplastic honeycomb materials onto which aluminum or fiber-reinforced thermosetting skins are applied. These materials have particularly high stiffness and modulus, but their cost is prohibitive due to the very high cost of honeycomb materials. Moreover, such materials do not lend themselves to the attachment of fasteners, hinges, and other hardware items; nor are they easily formable to other than strictly planar shapes.
In similar fashion, a variety of structures have been produced from polymer foam by first forming a foam structure, and then adhesively bonding either thermoplastic, metal, or thermosetting fiber reinforced skins onto the foam core. These processes are deficient in several aspects. First, because of the relatively long curing time of the adhesives, as well as fiber reinforced thermoset materials, production time is relatively long and therefore expensive. Second, because the polymer foam core is not completely encapsulated, it is subject to shear stress under bending which may result in premature failure unless the skins are made of great thickness in order to mitigate the bending stress. However, making the skins thicker, and therefore stronger, increases both the product cost as well as the product weight.
Glass mat reinforced thermoplastic materials (GMT) have been in use for several years now. These materials are manufactured by laying down numerous strands of glass fibers into a planar array, and needling these fibers with a needle board containing numerous barbed needles. The needling operation causes the fibers to intertwine, to break, and to assume a more random distribution. The mats thus produced have the appearance of a deep pile velvet material having a thickness of from about 3 or 10 mm. These needled glass fiber mats are then impregnated with a thermoplastic in a continuous double band press. The impregnation is done at such a pressure that a lofty (low density and unconsolidated) material is produced. This lofty GMT "intermediate" product may then be laid up into a shape suitable for thermoforming. The layup may contain from one to ten or more layers of GMT material. The GMT material is generally heated prior to placement into a mold, although heated molds may be occasionally used. The material is then fully densified under high pressure to form a very stiff, high modulus, fiber reinforced product.
Most common GMT materials are rather isotropic in nature, having been derived from randomly deposited fiber strands which are then intensively needled. However, if isotropic materials having enhanced strength in a given direction are required, strands of unidirectional glass fibers may be introduced on top of the randomly laid down strands, or alternatively, the strands may be laid down in elliptical rather than circular patterns. The higher proportion of fibers in one direction produces anisotropic or "unidirectional" GMT. Such composite materials have been utilized in a wide variety of relatively low cost, but dense and therefore relatively heavy load bearing structures.
For example, U.S. Pat. No. 5,122,398, discloses an energy absorbing bumper beam composed of a first, load bearing bumper support section which is designed to be attached to the vehicle frame, a core of expanded polyolefin thermoplastic foam for energy absorption characteristics, and an outer covering shell of another polymer. However, in the production of such bumper materials, it is necessary to form the three portions of the composite as separate components, and then bond them together adhesively. Thus, although the expanded polyolefin absorbing substrate is fully encapsulated, it is not fully encapsulated by a load-bearing GMT material, nor is it integrally encapsulated, i.e. encapsulated at the same time that the GMT material is molded. Rather, it is separately molded and adhesively bonded. The separate molding and adhesive bonding steps add additional cost and expense to the preparation of such bumper materials. As the covering layer is not fiber-reinforced, it has little tensile or compressive strength. The composite is not therefore sufficiently resistant to bending (shear) stress.
It would be desirable to be able to produce high modulus, high stiffness components having significant areal dimensions, yet having low weight. It would be further desirable to produce such components from readily available raw materials in a cost effective manner.